I didn't realize you had to install an earlier version with XP, before installing the latest version. This KBdocument says you can install the latest version, but an earlier version like 43.0 must be installed first:

I didn't realize you had to install an earlier version with XP, before installing the latest version. This KBdocument says you can install the latest version, but an earlier version like 43.0 must be installed first:

If I download and install Light Firefox_v43.0, will I have any problems installing 48.0 which is the latest version?

That sumo article is archived as it is outdated now. The issue was for during Firefox 43.0.2 till 48.0.2 the .exe's were signed (due to a Microsoft requirement) in a way that WinXP users with SP2 and not SP3 could not run the setups. Since it is not possible to tell by useragent if a WinXP user has SP2 or SP3 installed the 43.0.1 was served so they could then update to current Release at time. Vista had same issue if they did not keep Vista updated with at least a needed update.

The article was archived when Firefox 49.0 was Released as WinXP users still using SP2 could now install the setups fine since including the current 50.1.0 Release.

Note the future Firefox 53.0 and later will be dropping support for WinXP and Vista so the latest WinXP users can use will be the then Firefox 52.0 ESR Releases. Also Firefox 49.0 and later has dropped support of old CPU's that only supported SSE and not SSE2 flag.